Integrated circuitries or integrated circuits (ICs), respectively, are usually mounted in housings in order to protect the sensitive integrated circuitries from environmental influences. In the process, it is, however, to be observed as an unpleasant side effect that already the housing and mounting of the integrated circuitry in a housing imparts a substantial mechanical stress onto the semiconductor material and thus to the semiconductor substrate of the integrated circuitry. This in particular applies to low-cost housing forms implemented as mass items, like e.g. for such housing forms in which the integrated circuitry is moulded by a casting compound. This casting compound then solidifies by cooling the casting compound from a temperature of about 150° C.-185° C. to ambient temperature. As the semiconductor material of the integrated circuitry and the plastic casting compound material of the housing surrounding the integrated circuitry comprise non-matching thermal coefficients of expansion, the plastic material contracts more strongly when cooling to ambient temperature (room temperature) and therefore exerts a mechanical stress onto the semiconductor material of the integrated circuitry which is hard to predict and therefore not very well reproducible. The plastic material generally comprises a greater coefficient of thermal expansion than the semiconductor material of the integrated circuitry, wherein as a semiconductor material mostly silicon but also germanium, gallium arsenide GaAs, InSb, InP etc. is used.
The mechanical stress (or the mechanical stress, respectively) in the semiconductor material of the semiconductor substrate acting on the integrated circuitry may generally not be reproduced well because of the fact that the mechanical stress depends on the combination of the material used for the semiconductor substrate and for the compound and apart from that on the processing parameters, like e.g. the solidification temperature and solidification time of the compound of the housing of the integrated circuitry.
By different effects in the semiconductor material, so-called piezo-effects, like in particular by the piezo-resistive effect, the piezo-MOS-effect, the piezo-junction effect and the piezo-hall-effect, by a mechanical stress in the semiconductor material of the integrated circuitry also important electric parameters in the integrated circuitry are influenced. In the process, under the general term of “piezo-effects” in this connection the changes of electric parameters of the semiconductor material under the influence of a mechanical stress in the semiconductor material are designated. In particular with indirect semiconductors, like e.g. silicon, germanium, these piezo-effects are strongly developed. Semiconductors are referred to as indirect semiconductor materials in which the energy maximum of the valence band and the energy minimum of the line band are present at different crystal impulses. Direct semiconductor materials, like e.g. GaAs and InSb are semiconductors, however, in which the energy maximum of the valence band and the energy minimum of the line band are present at identical crystal impulses.
The mechanical stress in a semiconductor material may be regarded comparable to an elastic deformation work as a type of excitation energy, which must be added to the energy balance in the semiconductor material. In particular, a mechanical stress in the semiconductor material leads to a change of the band structure of the semiconductor. With indirect semiconductor materials this results in a separation of energy minima, which are actually identical in the stress-free case. This causes, as a further consequence, a new occupation of these energy minima with free charge carriers, wherein a main part of the charge carriers will take on the energetically more favorable state. As the curvature of the band edges, i.e. the edges of the energy bands of the free charge carriers in the semiconductor crystal, i.e. the line band edge for the free electrons and the valence band edge for the free holes, is also different in the different energy minima, different effective masses may be associated with the charge carriers in these energy minima, whereby their behavior with regard to the charge carrier transport is different. In this way, a mechanical stress in the semiconductor material causes the characteristics of the charge-carrier with regard to the charge-carrier transport to change, like e.g. movability, collision time, scattering factor, Hall-constant, etc.
Thus, the piezo-resistive effect indicates, how the specific ohmic resistance of the respective semiconductor material behaves under the influence of a mechanical stress. The piezo junction effect indicates how mechanical stresses in semiconductors leads to a shift of the energy level of the semiconductor material. From this, among others, changes of the characteristic curves of diodes and bipolar transistors result. The piezo Hall effect describes the dependence of the Hall constant of the semiconductor material on the mechanical stress state in the semiconductor material.
It thus becomes clear that with an exceedingly high mechanical stress, the electric functioning of the integrated circuitry may be affected, wherein initially generally only the electronic performance of the integrated circuitry will decrease. This decrease of the performance is for example to be observed in the form of an impairment of the modulation range, the resolution, the band width, the current consumption, the accuracy, etc., wherein it is to be considered that with a higher mechanical stress in the semiconductor material a partial or also complete functioning failure of the integrated circuitry may result which is generally still reversible, however. With an even further increased stress in the semiconductor material finally a crack formation in the semiconductor material will occur, wherein the semiconductor chip may finally break so that an irreversible damage of the integrated circuitry will occur. Typically, the junctions between the different stages of the impairment of the electronic performance of the integrated circuit occur gradually.
In the scientific publication “CMOS Stress Sensors on (100)Silicon” by R. C. Jaeger et al., in IEEE Journal of Solid-State Circuits, Vol. 35, No. 1, January 2000, pp. 85-94, analogue CMOS stress sensor circuits are presented based on the piezo-resistive performance of MOSFETs. The theoretical and experimental results should supply additional design rules for calculating and minimizing the sensitivity of conventional analogue circuits with regard to mechanical stresses in a semiconductor material which are caused due to the accommodation of a semiconductor chip in a housing.
The effects of mechanical stresses on the transistor performance are very important for manufacturers of modern integrated circuits, as by the different manufacturing steps and due to a plurality of housing processes including the chip mounting and encapsulation high values of mechanical stress in the semiconductor material may be caused. The caused mechanical stress may negatively influence the performance of both analogue and digital circuits or may also lead to a complete failure of the integrated circuitry.
It turned out that resistance-based piezo-resistive stress sensors represent a powerful aid for an experimental structure analysis of housed integrated circuitries. The values of integrated resistors are characterized before and after the housing process, wherein resistance values are used based on the piezo-resistive effect in order to calculate the mechanical stress in the material of the semiconductor substrate. If the piezo-resistive sensors are calibrated via a wide temperature range, also thermally caused mechanical stresses may be measured. Finally, also a complete mapping of the stress distribution across the surface of the chip using specially designed test chips may be obtained comprising an array of sensor rosettes.
The above-mentioned scientific publication thus refers to especially designed test chips with special stress sensors in order to experimentally detect mechanical stresses on a semiconductor chip and to indicate simulation and design rules for circuitries from the experimentally obtained results on a semiconductor chip, so that the mechanical stresses caused by the accommodation in a housing in the semiconductor material and the changes of the electrical characteristics of the circuitries associated with the same may be considered in the simulation and the design of the circuitries.
As the mechanical stresses are generally not well reproducible at an integrated circuitry in a semiconductor material, as they depend on the combination of the used materials and the processing parameters, like e.g. solidification temperature and solidification time of the compound mass of the housing of the integrated circuit, a unique characterization of any concerned parameters may generally not lead to the desired aim in the laboratory.
By a professional mounting process with the housing of the integrated circuitry (IC) it is therefore to be prevented that the mechanical stress load of the semiconductor material of the semiconductor chip takes on inadmissibly high values. In the process, primarily attention is only paid to the fact that the semiconductor material of the integrated circuitry does not break. Performance impairments occur, however, very product-specifically and may therefore not always be considered sufficiently in housing development.
Additionally, it is to be noted, that it is necessary again and again to perform small changes in the mounting process on the housing of the integrated circuitry. Thus, it may occasionally be the case that the deliverer for a casting compound has to be changed or that for certain reasons, like e.g. environmental protection measures, a conversion to a special casting material, e.g. to a halogen-free casting material, is to be performed. In practice, with such changes generally post-qualifications with regard to the mounting processes are performed when housing the integrated circuitry. Because these changes may concern many different products, however, in this case the post-qualifications are generally only performed on selected IC groups exemplary for the whole IC product family. The diverse influence possibilities of mechanical stress onto the functioning of a highly integrated circuitry are manifold, however, and are not easily apparent, so that it frequently occurs that with such a change the characteristics of a product become worse due to one of the above effects.
Particularly problematic is the above-explained problem with automotive products, i.e. products for the vehicle area, as these products are frequently subject to extreme temperatures or temperature fluctuations, respectively, but in case of sensors for security systems, like for example ABS sensors, airbag sensors, etc., have to function in a highly reliable way over their complete life. From this group of products the automotive sensors are also very prone to piezo-interactions, as these automotive sensors are frequently subjected to specially extreme temperatures due to their use and need to work a lot more accurately as sensors in security systems than it is the case with simple digital integrated circuitries.
The above-discussed problem is, however, even further increased when the integrated circuitries housed in a housing are again assembled in a module. In vehicle technology this module must be cast frequently in order to protect the same from environmental influences, like e.g. from motor oil, dust, salt water, etc. When casting, again substantial mechanical stresses may be caused in the semiconductor material of the integrated circuitry. It is, however, generally not possible with a reasonable technical effort to keep the manufacturing parameters of those processing sequences sufficiently reproducible, so that also in the course of the production of the above-mentioned module substantial fluctuations of the mechanical stresses in the semiconductor material of the integrated circuitries on the semiconductor chip may result. Thus, the procedure illustrated in the above-mentioned publication for the simulation and assessment of mechanical stresses in a semiconductor material may neither be applied satisfactorily.
Further, in connection with the assessment of the influence of mechanical stresses on the electric characteristics of integrated circuitries it should be considered, and in particular when the mechanical stresses are caused by the accommodation in a housing, that the piezo-effects are very strong at low temperatures. This may on the one hand be attributed to the fact that the piezo-resistive constants increase with a decreasing temperature, wherein on the other hand the stress of the casting materials solidified in the hot state increases with different coefficients of expansion. As the modules already comprise a considerable mass, generally several 100 grams, it is further not possible basically due to the very high technical effort and therefore for cost reasons, to test all manufactured modules at the minimal operation temperature with regard to the problem regarding piezo-effects at the output of the manufacturing line. Due to the considerable mass of for example several 100 grams, a relatively long time period would be required in order to cool every module completely to the minimal operation temperature.
Apart from that, generally during the production of the modules, an attempt is made not to apply temperatures below dew point, as thereby condensed water may result and the same may cause leakage currents in exact electrical measurements. Therefore, before packaging the module the same has to be sufficiently annealed. This would even be required in order to prevent a corrosion of the module within the packaging.
Due to the above-illustrated problem with regard to the stress sensitivity of integrated circuitries, therefore within the product qualification the influence of the piezo-effects is observed in so far that based on individually selected integrated circuitries so-called process split-less worst-case cases with regard to the piezo-effects are generated and the integrated circuitries or circuit modules, respectively, are examined. Accompanying the manufacturing, both at the semiconductor manufacturer and at the module manufacturer sample tests are performed, as it would be extremely cost- and time-consuming to perform corresponding functioning tests on all completed semiconductor devices or modules, respectively.
As mechanical stresses in the semiconductor material of an integrated circuitry were found to be a reliability risk, it was now started to monitor the mechanical stress. The approach according to the prior art is now to detect those electric characteristics of the integrated circuitry which are especially prone to the piezo-effects, wherein those characteristics are then examined in a functioning test after mounting the module or accommodating the integrated circuitry in a housing, respectively. This may for example be performed in semiconductor chip manufacturing, wherein it is generally not possible, however, to perform such cost- and time-consuming electronic tests after the manufacturing of the module, as the integrated circuitry (IC) is already located in a complex system with numerous other components and is not individually acceptable any more.
Additionally, piezo-effects may spread to a system in hardly (or not at all) predictable ways and generate detrimental effects. In particular, mechanical stresses in a semiconductor chip may only cause relatively small observable changes in the integrated circuitry, wherein, however, in connection with other system components, a failure or a performance decrease of the integrated circuitry or the circuit module, respectively, may then result.
Additionally it is to be noted, that according to the prior art still a temperature derivative action for those effects has to be calculated, as those effects may not or cannot be measured at the minimum operation temperature of the integrated circuitry (as explained above). Due to the occurring non-linearities of the used circuit structures this is, however, mainly not sufficiently possible. In addition it is to be noted, that such temperature derivative actions are also again flawed with tolerances.
From the above disclosures it becomes clear that it is very cost and time consuming in chip or module manufacturing, respectively, to examine the functioning of all integrated circuitries accommodated in a housing or module, respectively, with regard to the influence of mechanical stresses in the semiconductor material.
In the simulation of the mechanical stress values in the semiconductor material of an integrated circuitry, which is provided to make predictions with regard to the functioning of the circuitry and to be able to suitably adapt the circuit design of the same, it should be considered, however, that already slight changes of the housing materials, the manufacturing sequence or of individual processing steps may invalidate the practicality and the use of the simulated values.